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Use some tools to check out the interface and see if there is an active link
on it. Start with typing in /sbin/mii-tool. That command will show if you
even have an active link and what the speed/duplex is. Any chance any
package or upgrade was installed that might have changed the adapter? Some
updates have been known to change Intel Pro adapters from e100 to e1000 and
things stop working. Also check to see if the module is loaded using
lsmod. What type of adapter is this? What type of server/system? The
solution may be as simple as checking in /etc/sysconfig/network-scripts and
look for the corresponding file. Assuming eth0 then you should have a file
named ifcfg-eth0. Check the contents of that file (if it exists) to ensure
the the line ONBOOT=3D is set to "yes" and that the DEVICE matches the correct
device. For example ifcfg-eth1 should contain an entry that states
DEVICE=3Deth1. If the required file does not exit, these entries should get
you by for an ifcfg-eth0 file:

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